1. Field of the Invention
This invention relates generally to an electrophotographic imaging method, and more particularly, to an electrophotographic imaging method in which an image is formed by directly contacting a liquid developer on the surface of an organic photoreceptor.
2. Description of the Related Art
In electrophotography, the surface of a photoconductive element is selectively exposed to light to form a latent image, and a difference in electrostatically charging density between an exposed area and non-exposed area is generated to form a visible image by an electrostatic toner containing pigments or thermoplastic components.
In electrotography, wet type developing using liquid developers, as disclosed in U.S. Pat. Nos. 2,907,674 and 3,337,340, has been well known for a long time, and liquid developers are capable of producing very high resolution images because of the small particle size, ranging to submicron size.
However, the wet type developing method has not been widely used because of several drawbacks, such as flammability or offensive odor due to a petroleum-based solvent as a main component of liquid developer. Thus, a dry type developing method using dry powder developers has been generally regarded as one of representative developing methods.
Owing to the advantage of the capability of producing very high resolution images, much attention has been being paid again to wet type developing in recent years.
In wet type developing, an electrostatic image is formed on the surface of a photosensitive layer and is then moved to another surface. The surface is wetted using a liquid carrier containing a pigment and having electrostatic resistance enough to suppress damage of the electrostatic image, thus achieving development.
In wet type developing using liquid developers, inorganic photoreceptors such as amorphous selenium have been used conventionally, but when this method was applied to organic photoreceptors, the following problems were found.
If the surface of an organic photoreceptor is formed of a charge transport layer containing a binder, e.g., a polycarbonate-based resin or acryl-based resin, and a low molecular weight compound, i.e., a charge transport material, the charge transport layer forming materials are soluble in an aliphatic hydrocarbon-based solvent of a liquid developer. The liquid developer is prepared by dispersing pigment particles in an aliphatic hydrocarbon-based solvent.
Thus, when the liquid developer directly contacts an organic photoreceptor, the organic photoreceptor erodes by dissolving in the solvent, causing cracking or lowered photosensitivity, or resulting in contamination of the developer by photoreceptor components.
To overcome the above-described problems, research into photoreceptors having good durability with respect to liquid developers is being actively carried out. There are approximately three ways which have been proposed for attaining such organic photoreceptors, as follows:
(1) Polymerizing photoreceptor components, e.g., charge transport materials to prevent the same from being dissolved in solvent;
(2) Providing an overcoat layer having superior developer resistance to prevent solvent penetration into the photosensitive layer; and
(3) Improving the developer resistance of a binder to prevent solvent penetration into a photosensitive layer.
For example, U.S. Pat. No. 5,030,532 is classified as (1), but this method includes disadvantages, for example, the low number of available polymeric charge transport materials with superior solvent resistance and a lack of availability of a common resin, which increase the material cost substantially. U.S. Pat. No. 5,368,967 is classified as (2); nevertheless, the overcoat requires a complicated procedure for preparation and should be a thin layer for good electrical performance, so the photoreceptor includes a difficulty in obtaining a physically durable overcoat. U.S. Pat. No. 5,545,499 is classified as (3), but this method has the disadvantage of finding a photoreceptor which has enough solvent resistance, and so far a suitable photoreceptor has not been found.
Japanese Patent Laid-open Publication Nos. Hei 5-297601, 7-281456 and 10-20515 disclose an organic photoreceptor using a polyester resin having a biphenyl fluorene repeating unit in the main chain as a binder.
According to the above-cited patents, various attempts based on general electrophotography were made to improve mechanical durability by using particular polyester resins, but the applicability of liquid developing was not taught. Also, compared to conventional resins, the resins disclosed in the above-cited patents have poor electrical properties, and have not been practically used as photoreceptor materials.